So... In the version I'm currently using - the 4.14.2 - there are two files to modify in the module, to disable some checks and display the upgrade button:
- /modules/autoupgrade/classes/Twig/Block/UpgradeChecklist.php
- /modules/autoupgrade/classes/Twig/Block/UpgradeButtonBlock.php
In those files you'll need to update some boolean values, and disable a if condition in the UpgradeButtonBlock.php. After that, you'll be able to run the upgrade successfully...